The present invention relates to test instruments for measuring the impedance match between an antenna and a RF transmitter, and more particularly to a self contained pocket size antenna match indicator for providing a visual readout of voltage standing wave ratio (VSWR).
U.S. Pat. No. 4,110,685 to Leenerts is illustrative of a prior standing wave ratio measurement instrument useful for matching the impedance of a transmitter and an antenna. Two directional couplers are inserted in the transmission line to produce an output voltage indicative of forward and reflected RF power. The voltages are applied to analog circuitry for generating a signal proportional to the return loss. The signal is coupled to an analog display meter calibrated to provide a direct reading of the VSWR.
Measurement instruments of the type disclosed in the Leenerts patent are inserted in the transmission line between the transmitter and antenna, and rely on the transmitter as the source of RF power. Such arrangements often require the use of moderate to high RF power levels (i.e. +30 dBm and higher) to compensate for transmitter impedance peculiarities and instrument sensitivities, thus presenting potential serious hazard to personnel while tuning or adjusting the antenna. The prior art instruments present other difficulties in working at antennas located at remote ends of a transmission line. For example, the equipment is bulky and not well suited for use in adverse weather conditions, remote or poor access areas, or in darkness. Also, the measurement equipment is susceptable to extraneous RF signal radiations.
In view of the foregoing, it would be highly desirable to have a simple and rapid means of checking or adjusting the match condition of an antenna which does not rely on the RF transmitter as the signal source. What is needed is an antenna match instrument having a self contained low level RF power source that could be used independently of the transmitter. It would further be desirable to provide such a wholly self contained antenna match indicator that is inexpensive and which does not require a high degree of technical skill or knowledge to operate.
It is therefore an object of the present invention to provide a small, inexpensive antenna match indicator having a self contained low level RF signal source.
It is another object of the invention to provide a VSWR indicator which is ideally suited for use in remote or poor access areas and which does not impair or present a hazard to the operator due to size, complexity of operation, or power level.
It is yet another object of the invention to provide a VSWR indicator of sufficiently low power level that it will not cause significant RF radiation from an antenna under test.
It is another object of the invention to provide a VSWR indicator having an easy to read digital display readout comprising individual light emitting diode (LED) display segments indicative of direct measured VSWR values.
It is another object of the invention to provide a VSWR indicator having a moving dot type display to provide a readily ascertainable indication of a good or bad antenna match even in poor visibility.